This application is a continuation-in-part application of application U.S. Ser. No. 08/936,317, filed Sep. 24, 1997 now abandoned.
The present invention relates to the art of pumping devices and cardiac prosthesis. More particularly, the present invention relates to motor-driven rotodynamic pumps for use as blood pumps in the human body and to control systems and techniques for such blood pumps.
There has been much effort recently in the medical community to develop artificial devices and systems that are capable of assisting or completely assuming the cardiac function in patients having limited cardiac function or who have undergone removal of a diseased natural heart. Some of this effort has focused on duplicating the pulsatile mechanical action of the natural human heart. On the other hand, it has been recognized that human life may be sustained with non-pulsatile blood flow in the circulatory system. Accordingly, recent research has included the evaluation of non-pulsatile pumping devices, which provide a continuous flow of blood to the circulatory system, as prosthetic devices for assuming or assisting cardiac function. Typically, these non-pulsatile pumps take the form of rotodynamic blood pumps, also known as continuous flow blood pumps, centrifugal blood pumps, mixed flow blood pumps, or axial flow blood pumps. Rotodynamic pumps offer the advantage of reduced size and weight, simpler design, increased dependability and low cost compared to positive displacement or pulsatile pumping devices used as implantable pumps.
In the human body, the peripheral vascular resistance and venous “tone” are controlled by the body according to the needs of the body's organs. Blood vessels constrict (vasoconstriction) and expand (vasodilation) in response to neural impulses associated with blood demand required by the body's organs. This action results in pressure and flow variations within the circulatory system. In a sense, the natural heart is the servant of the circulatory system and the amount of blood pumped is dependent on the requirements of the body. That is, the cardiac output (the volume of blood delivered by the heart within a given time period) is equal to the venous return (the volume of blood returning to the heart within that same time period). The human heart is characterized by intrinsic control that responds to changes in demand for blood flow by the circulatory system. Illustrative of this characteristic is the fact that extrinsic control implements are not necessary when a human heart is transplanted and no direct neural connection is required for the transplanted heart to assume the cardiac function in the host body.
Rotodynamic pumps typically operate or are controlled to maintain a defined pressure difference between the pump inlet and outlet. Usually, pump controllers do this by maintaining a set impeller speed. The performance characteristics of a pump are often expressed by a performance curve which depicts the relationship between the pressure differential across the pump and the pump flow for a given pump operating speed.
The use of rotodynamic blood pumps as cardiac prosthesis presents unique problems with regard to the interaction between the pump and the human circulatory system. Compared to the natural heart and some artificial hearts, conventional rotodynamic pumps are not as apt to respond correctly to changes in pressure and flow induced by the human circulatory system. This is due in part to the fact that, unlike the natural human heart, rotodynamic pumps have no inherent sensitivity to inlet pressure (preload) or outlet pressure (aflerload). When pump speed is maintained at a controlled value, pump flow does not change appreciably unless there is a change in the pressure difference between the inlet and outlet—the pressure differential across the pump. Thus, in the case of a pump being used as a blood pump, a change in the inlet pressure, i.e., the venous return path of the circulatory system, which is accompanied by a like change in the outlet pressure, i.e., the pressure at the pump outlet, may occur without any significant change in the pump flow since the pressure differential across the pump remains constant. This contrasts sharply with the operation of the natural heart, in which a significant increase in flow is usually associated with an increase in venous pressure, with only a small and frequently transient effect from systemic pressure. Whether a change in pressure difference is caused by an increment in the inlet pressure or the outlet pressure is not particularly critical to the rotodynamic pump. However, with regard to the physiological system, a five or ten mm-Hg preload pressure change has a different physiologic significance than an equal amount of afterload change. As a consequence, if the outlet pressure falls to a low level, an inappropriately designed and/or controlled rotodynamic pump may urge flow through the system until the inlet pressure falls to a correspondingly low and perhaps dangerous level, where upstream vascular structures may collapse from lack of blood pressure. Conversely, if the outlet pressure becomes high, the inlet pressure might rise a similar amount, and, in extreme cases, the direction of flow might even reverse. The change, which is compensatory from the pump's point of view, is potentially maladaptive relative to the needs of the physiologic system being supported. In conventional pump constructions, the degree of maladaptivity of the pump is an inherent result, in part, of the nature of the performance curve associated with known pump designs. This problem may be exacerbated if the natural heart retains some contractile function, causing the artificial pump pressure difference to oscillate between very low levels during natural heart systole, and high levels during diastole. Within one heartbeat the system may experience excessive forward pumping, and reversed flow.
In clinical practice today, rotodynamic pumps are controlled by external consoles, and an operator increases or decreases speed according to medical judgement. Furthermore, most clinical cases to date with rotodynamic pumps have been done with external pumps which require long inflow and outflow cannulae. These cannulae contribute a relatively large pressure drop between the pump and the physiological system, making physiologic pressure swings a smaller part of the total resistance value resulting from the cannulae and physiological system combined. It has been proposed to use a system of pressure or flow transducers, in cooperation with the pump and a control algorithm, to produce a closed loop feedback controlled system for pump flow. However, such techniques or devices are often complex and offer no economically feasible solution to the problems of providing low cost and dependable blood pumping systems. It has also been proposed to control these rotary pumps by measuring the motor electrical current, and making speed adjustments based on assumptions regarding the relationship between flow, current, and speed. These protocols require additional logic and fallback positions for instances where the assumed relationships are not valid.
It would, therefore, be desirable to provide a blood pumping system that alleviates the aforementioned problems such that external sensors and control implements are not necessary for the pump to maintain an acceptable output and preserve the integrity of the circulatory system over long periods of time, a wide level of variation in residual ventricular activity, and a broad range of patient activity levels.